1. Field of the Invention
The present invention relates to an electronic apparatus comprising a signal transmitting print wiring plate and a signal receiving print wiring plate each incorporating a digital clock circuit and having a ground conductive member connected to each other on a conductive device frame through a cable including a signal wire and a ground wire for a signal transmission.
2. Related Background Art
Conventionally print wiring plates have been used for various types of electronic apparatuses such as a still camera, a video camera, and other electric devices. In a configuration in which a print wiring plate is mounted on an electronic apparatus, there are suggested various technologies for reducing noise.
For example, there are disclosed technologies in which a ground of a print wiring plate and a conductive device frame are grounded with resistances in Japanese Patent Application Laid-Open No. 8-64984 and Japanese Patent Application Laid-Open No. 7-225634. These conventional technologies in which the ground of print wiring plate and the conductive device frame are grounded with resistances are suggested to suppress radiated noise caused by a standing wave generated on a substrate.
Recently, however, an electronic apparatus rather rarely has a configuration in which only a single print wiring plate is mounted in the device frame and mostly it has a plurality of print wiring plates connected with each other through a cable. In other words, in recent years, due to advanced functions of various electronic apparatuses as described above and complicated electronic circuits being mounted, there are plenty of configurations in which a print wiring plate is divided into a plurality of units and they are connected through cables for a signal transmission.
Practically the configuration in which the cables are connected to the print wiring plates is far susceptible to a problem of unnecessary noise radiation (EMI: Electro magnetic interference) in comparison with a configuration in which a single print wiring plate is provided in a frame. In such a case, the main noise radiation source is often a cable.
On the other hand in an electronic apparatus having the above configuration, if each connected print wiring plate has different impedance characteristics from those of each cable signal line, a reflection of an electric signal occurs due to a mismatch of the impedance characteristics, thus causing a radiated noise generation or a disorder of a signal waveform.
To solve this problem, conventionally there have been suggested various technologies. For example, there is disclosed a configuration in which radiated noise is reduced by matching impedance characteristics of a signal line of a cable, load impedance of a transistor, and receiving terminal impedance of a substrate with each other in Japanese Patent Application Laid-Open No. 5-27697. This technology is used for reducing radiated noise by means of matching with impedance of a signal line.
In a configuration in which two print wiring plates are connected through a cable on a conductive frame for a signal transmission, however, current flowing through the ground wire may become a main cause of radiated noise rather than current flowing through a signal line.
While the characteristics of current flowing through the ground wire depend upon the impedance characteristics determined by a relationship between the ground wire and the frame, normally a ground conductive member of a print wiring plate is extremely different in shape from a ground conductive member of a cable and therefore the impedance characteristics between the print wiring plate and the cable are significantly different from each other, by which a junction between the print wiring plate and the cable becomes a mismatch point of the impedance characteristics, thereby reflecting the current flowing through the ground conductive member and then a resonance peak of the radiated noise caused by the reflection may lead to a problem on standards or on practical use.
Therefore it is a first object of the present invention to provide an electronic apparatus which solves the above problems of the prior art and reduces radiation from a cable mainly causing the radiation of radiated noise.
It is a second object of the present invention to provide an electronic apparatus which reduces radiated noise generated by a reflection of signal current flowing through a ground conductive member of a cable at a conductive junction between a print wiring plate and the cable which is a mismatch point of impedance characteristics.
According to a first aspect, the present invention which achieves the first object related to an electronic apparatus, comprising: a conductive device frame, a signal transmitting print wiring plate incorporating a digital clock circuit and having a ground conductive member, a signal receiving print wiring plate incorporating a digital clock circuit and having a ground conductive member, a cable including a signal wire and a ground wire for a signal transmission and electrically connecting the signal transmitting print wiring plate to the signal receiving print wiring plate on the conductive device frame, and an impedance element electrically connecting an arbitrary point on the ground conductive member on the signal receiving print wiring plate to the conductive device frame.
In the above arrangement, the impedance element has circuit parameters equivalent to the impedance characteristics between the ground wire of the cable and the conductive device frame.
In the first aspect of the invention, there can be further arranged a low-impedance element electrically connecting the ground conductive member of the signal transmitting print wiring plate to the conductive device frame.
In the above first aspect of the invention, it is preferable to have an arrangement such that a point on the ground conductive member of the signal receiving print wiring plate connected with the conductive device frame by the impedance element is located in the vicinity of a connection point between the receiver print wiring plate and the cable. The above cable comprises, for example, a coaxial cable.
According to a second aspect, the present invention which achieves the second object related an electronic apparatus, comprising: a conductive device frame, a signal transmitting print wiring plate being mounted on the conductive device frame, incorporating a digital clock circuit, and having a signal conductive member and a ground conductive member, a signal receiving print wiring plate being mounted on the conductive device frame, incorporating a digital clock circuit, and having a signal conductive member and a ground conductive member, and a cable including a signal conductive member for a signal transmission and a ground conductive member and electrically connecting the signal transmitting print wiring plate to the signal receiving print wiring plate on the conductive device frame.
In the above arrangement, the impedance of the signal conductive member and the ground conductive member of the cable depending upon a correlation with the conductive device frame is balanced with the cable-side impedance of the signal conductive member and the ground conductive member of the signal receiving print wiring plate depending upon a correlation with the conductive device frame.
This arrangement enables the electronic apparatus according to the second aspect of the invention to suppress signal reflections at the junctions between the signal conductive member and the ground conductive member of the cable and respective signal conductive members and ground conductive members of the signal transmitting print wiring plate and the signal receiving print wiring plate, thus reducing the radiated noise caused by the signal reflections.
In the electronic apparatus according to the second aspect of the invention, the impedance can be balanced by, for example, utilizing a distance from the conductive device frame to the signal conductive member and the ground conductive member of the cable which is different from a distance from the conductive device frame to the respective signal conductive members and ground conductive members of the signal transmitting print wiring plate and the signal receiving print wiring plate.
Additionally as means for the impedance balance, a high dielectric constant member can be inserted between the cable and the conductive device frame.
Furthermore in the electronic apparatus according to the second aspect, the arrangement may be such that the impedance of the ground conductive member of the cable is balanced with the cable-side impedance of the respective ground conductive members of the transmitter print wiring plate and the receiver print wiring plate and that the impedance of the signal conductive member of the cable is balanced with the cable-side impedance of the respective signal conductive members of the transmitter print wiring plate and the receiver print wiring plate.
In the second aspect, the cable may comprise a cable including a signal conductive member and a cable including a ground conductive member. Furthermore, a coaxial cable can also be used as a cable.